This invention relates to apparatus for converting energy present in surface waves of bodies of water into useful electrical energy and, in particular, to the design of floats (or shells) for use in wave energy converters (WECs) to improve the power generation efficiency and survivability of the WECs.
Various wave energy converter (WEC) systems are known. For example, reference is made to U.S. Pat. No. 6,921,904 titled “Wave Energy Converter Utilizing Pressure Difference”; U.S. Pat. No. 6,617,705 titled “Protector Arrangement For Natural Energy Power Generation Systems” and U.S. Pat. No. 7,141,888 titled “Antirotational Structures For Wave Energy Converters” all assigned to the assignee of the present application, and the teachings of which are incorporated herein by reference.
Known WEC systems generally include a “float” (or “shell”) and a “spar” (or “shaft” or “column” or “piston”) which are designed to move relative to each other to convert the force of the waves into mechanical energy. In these systems, the float is generally depicted or referred to as the moving member and the spar as the non-moving or mechanically grounded member. But, the opposite may be the case. Alternatively, the spar and float may both move relative to each other.
As shown in FIGS. 1 and 1A, a WEC generally includes a float 10 (also referred to as a shell) and a spar 20 (also referred to as a shaft or column), which are intended to move relative to each other in response to the waves, and a power-take-off device (PTO) 30 coupled between the float and spar to convert their relative motion into a useful form of energy. The PTO device may be any device capable of converting the relative motion between the float and spar into electrical energy or mechanical energy (e.g., performing some other type of useful work such as desalinating seawater).
In general, to obtain the most power efficient system, it is desirable that the float 10 of the WEC be designed such that the displaced volume of the float is preferentially located close to the waterplane of the float. The neutrally buoyant or water plane of a float, represented by dashed line 300, may be defined as the “still water level” of the float in the absence of action by the PTO. The water surface plane 301 is intended to show the instantaneous water level. For example, FIG. 1A shows the top surface 10a and bottom surface 10b of the float 10 to extend in a horizontal direction, parallel to each other, and both surfaces have essentially the same diameter and surface area. This generally flat bottom structure of the float provides good power generating efficiency because wave-induced hydrodynamic force, (which may be approximated as the change in buoyant force), is what enables power generation. The change in the buoyant force on an object is defined as ρgΔV, where ρ is the density of water, g is the acceleration due to gravity, and ΔV is the change in displaced volume; where displaced volume of a body is defined as, and refers, to the volume of the portion of said body that is below the instantaneous water surface 301. For small displacements of the water surface 301 where the water surface is displaced by a distance h from the water plane, the change in displaced volume ΔV is hA, where A is the waterplane area.
A problem with the floats shown in FIGS. 1 and 1A is that its extended flat (“pancake”) bottom surface is subjected to “wave slamming events” which may also be referred to as “water slamming”. For example, if the bottom of the float is lifted above the water surface due to large waves, or any other cause, when the float drops and hits the water surface, the flat bottom will be subjected to large impact forces which threaten the survivability of the float.
In addition, the float may be subjected to dangerous bending moments. For example, if a large force (due to water impact) acts on the bottom of the float near the outer edge of the float, the effective lever arm (to the central point of the float) will be the float radius. A large impact force on the bottom of the float, acting with the full float radius as a lever arm, may lead to a dangerous bending moment (force times distance) on the float-spar connection mechanism.
Thus, to increase the survivability of the float and to keep it safe from wave slam, it is undesirable for the float to present a large surface area that is substantially flat or substantially horizontal to the water surface.
Referring to FIG. 2, there is shown a float 100 having an underside 100b whose cross section is shaped like an inverted truncated cone or triangle. A line 300 indicates the waterplane of the float and a line 301 represents the surface of a wave. In response to waves, float 100 moves up and down relative to a spar 20 which may be terminated at its lower submerged end with a heave plate 22. [Although not explicitly shown in all the figures, a spar 20 used to practice the invention may be generally anchored, as shown in FIG. 1, or connected to a heave plate as shown in FIG. 2. Also, in WECs embodying the invention, a PTO is coupled between a float and a spar as shown in FIG. 1, although not shown in the remaining figures.] The float 100 is designed to reduce the problem of “wave slam”. However, the single inverted truncated cone shaped water-displacing portion of float 100 is not optimized with regard to the requirement for producing maximum power. This is demonstrated in FIG. 2 by showing that the effective area of the float capable of responding to the waves has decreased from Da to Db. That is, the effective area of the submerged portion of the float will be a function of the surface area at 101b. For the truncated cone, the surface area to be acted upon by surface wave 301 would (neglecting the central opening for the shaft) be approximately
            π      ⁡              (                              D                          b              ⁢                                                                            2                )              2    ,where Db is the diameter of the underside below the water line and along the wave front. For the flat bottomed toroid the surface area at 101a would be approximately
            π      ⁡              (                              D            a                    2                )              2    ,where Da is the diameter at and above the water line. Thus, although the truncated cone shaped float, shown in FIG. 2, functions to decrease loads associated with slamming, the inverted truncated cone underside has the disadvantage of providing less buoyant-force response for small waves and only limited protection against wave slamming.
Thus there exists a problem of producing a float with increased survivability and satisfactory power generating capability. This problem is addressed in accordance with floats embodying the invention.